Currently, information is displayed using assembled sheets of paper carrying permanent inks or displayed on electronically modulated surfaces such as cathode ray displays or liquid crystal displays. Printed information cannot be changed. Electrically updated displays are heavy and expensive. Other sheet materials can carry magnetically written areas to carry ticketing or financial information, however magnetically written data is not visible.
Media systems exist that maintain electronically changeable data without power. Such system can be electrophoretic (Eink), Gyricon or polymer dispersed cholesteric materials. An example of electronically updateable display can be found in U.S. Pat. No. 3,600,060 issued Aug. 17, 1971 to Churchill that shows a device having a coated then dried emulsion of cholesteric liquid crystals in aqueous gelatin to form a field responsive, bistable display. U.S. Pat. No. 3,816,786 discloses a layer of encapsulated cholesteric liquid crystal responsive to an electric field. The electrodes in the patent can be transparent or non-transparent and formed of various metals or graphite. It is disclosed that one electrode must be light absorbing and it is suggested that the light absorbing electrode be prepared from paints contains conductive material such as carbon.
Fabrication of flexible, electronically written display sheets is disclosed in U.S. Pat. No. 4,435,047 issued Mar. 6, 1984 to Fergason. A substrate supports a first conductive electrode, one or more layers of encapsulated liquid crystals, and a second electrode of electrically conductive ink. The conductive inks form a background for absorbing light, so that the display areas appear dark in contrast to non-display areas. Electrical potential applied to opposing conductive areas operates on the liquid crystal material to expose display areas. Because the liquid crystal material is nematic liquid crystal, the display ceases to present an image when de-energized.
The Fergason patent discloses the use of nematic liquid crystal, which absorbs light and does not maintain an image in the absence of a field. Dyes in either the polymer encapsulant or liquid crystal are used to absorb incident light. The dyes are part of a solution, and not solid sub-micrometer particles. The patent further discloses the use of a chiral dopant in Example 2. The dopant improves the response time of the nematic liquid crystal, but does not operate in a light-reflective state.
U.S. Pat. No. 5,251,048 discloses a light-modulating cell having a polymer-dispersed chiral-nematic liquid crystal material. The chiral-nematic liquid-crystal material has the property of being electrically driven between a planar state reflecting a specific spectrum of visible wavelength of light and a light scattering focal-conic state. Chiral-nematic liquid-crystals, also referred to as cholesteric liquid crystals, have the capacity of maintaining (in a stable state) one of a plurality of given states in the absence of an electric field. Black paint is applied to the outer surface of a rear substrate in the cell to provide a light-absorbing layer outside of the area defined by the intersection of segment lines and scanning lines.
Cholesteric liquid crystals reflect a portion of the visible spectrum when in a reflective state. It is preferable that the reflective state have neutral color balance. It would be useful to provide cholesteric displays exhibiting neutral density in the reflective state. It would be useful for such display to be fabricated using simple, low-cost processes.